With the proliferation of equipment such as digital cameras, scanners and computers digital imagery has become ubiquitous. The world wide web has also contributed its share of digital imagery. Frequently, whether because of limitations of technology or of skill, images are of less than optimal quality. While an expert may have the skill and talent to adjust and correct such images, a consumer without training in the image processing art finds it more difficult. Such a person could benefit from automated means of image quality improvement. Even the expert can benefit from such means since they can be a way of saving time. Among the image properties requiring correction is saturation. A properly saturated image has vibrant or vivid colors, which, however, maintain a natural look. Though it is perhaps more common to encounter undersaturated images, oversaturated images occur with some frequency. Most usually, the latter are the result of misapplied corrections performed with image editing software. Any automated method of image saturation adjustment must, therefore, cope with both oversaturated and undersaturated images. At present, no such method is available.
European. Pat. 1 017 241, and the references disclosed therein, describes a method of correcting image saturation following a change in the image tone curve (i.e. image contrast). However, the disclosure is concerned with restoring the saturation to an appearance similar to that prior to the change in contrast, rather than with improvement of the saturation. U.S. Pat. No. 5,982,926 discloses, along with other operations, the transformation of image chrominance to correspond to the entire dynamic range of chrominance values. This process cannot correct oversaturated images and, indeed, may oversaturate well adjusted images. U.S. Pat. No. 4,812,902 describes a method of increasing chrominance using a non-linear characteristic curve, which has as its objective an increase in the saturation of low-saturation pixels without an increase in the saturation of highly saturated pixels. Such a process cannot reduce image saturation and, by boosting the saturation of areas of low chrominance, can contribute to a perception of color imbalance. This problem is addressed in U.S. Pat. No. 5,390,034, which accomplishes chroma correction by taking into account both luminance and chrominance to determine correction parameters. The parameters are chosen to increase chrominance where it is high and to leave regions of low chrominance and luminance unchanged, or even to force their complete desaturation. This recipe does not allow image saturation as a whole to be lowered. U.S. Pat. No. 5,517,335 describes a method of increasing saturation of halftone images by removing an increment from the least intense primary color and adding it to the most intense primary color. Again, such a procedure can only increase the saturation of an image though the changes do not correspond well to human perception of saturation. A very similar process is described in U.S. Pat. No. 5,790,282, wherein the color increment can be of both positive and negative sign, so permitting desaturation. However, this patent does not teach any means of automatically adjusting saturation. A method of automatic saturation adjustment is described in a paper entitled “Adaptive Quality Improvement Method for Color Images” (A. Inoue and J. Tajima, NEC Res. Dev., 35, 180 (1994)). Here, the saturation of all image pixels in the HSV (hue-saturation-value) color space is linearly stretched such that the most saturated pixel acquires the maximum saturation it is possible to display in this space. This approach can only increase saturation and is, moreover, not robust since the adjustment depends on the saturation of but a single pixel, which may be subject to noise. U.S. Pat. No. 5,450,217 describes a more reliable and sophisticated variant of this type of correction that is based on an average saturation within certain value (i.e., lightness) bounds and compensation of image luminance for a change induced by the correction of saturation in the HSV color space. If the average saturation is less than a target value it is increased, otherwise it is left unchanged. Accordingly, this method does not provide a means of reducing excessive saturation. WO 97/01151 claims a “procedure for automatically maximizing the visual quality of an image in electric format, characterized in that the following operations are performed automatically: the image is converted into a chromaticity coordinate format; the probable grayness balance is determined for the image by a statistical method from the video signal and the grayness balance of the image is adjusted to correct it; the color reproduction of the image is adjusted; and the maximized image is converted to from the chromaticity coordinate format back to its original format”. While the Lsα chromaticity space discussed in the patent includes a saturation coordinate, s, no explicit mention is made of saturation adjustment. Moreover, the patent does not disclose any specific embodiment of the invention that would enable the adjustment of “colour reproduction”.
Accordingly, there remains a need for a method of automatically adjusting the saturation of an image that will reliably provide a desirable saturation whether the initial image is oversaturated or undersaturated.